Water vapor diffusion in asphalt mixtures under different relative humidity differentials

A linear model was established between the relative humidity differential and the total diffused mass based on the real gas law. An inverse proportionality was identified between the relative humidity differential and the diffusivity in both radial and axial directions. The determined diffusivity values were further converted to the corresponding diffusivity values under 1 atmosphere, which were in agreement with the diffusivity data reported in the literature. A linear model was derived for the relationship between the relative humidity differential and the diffusivity under 1 atmosphere, which indicated faster water vapor diffusion in asphalt pavements when subjected to a larger relative humidity differential between the subgrade and the atmosphere. The diffusivity in the radial direction was always larger than that in the axial direction despite the variation of the relative humidity differential. This fact implied the anisotropic distributions of air voids and aggregates in the asphalt mixture specimens, which assisted water vapor in diffusing more easily in the radial direction than in the axial direction.